This model zoo repo if for old tools only (DNNDK)

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README needs to be updated to indicate that this repo is intended for use with older DNNDK tools. Users of the latest Vitis AI framework should be re-directed to https://github.com/Xilinx/Vitis-AI/tree/master/AI-Model-Zoo. Or users in general should be redirected to the Vitis-AI repo since the DNNDK flow is now deprecated.